1. Field of the Invention
The present invention relates to a thermal separating process between at least one gaseous stream ascending in a separating column containing a sequence of mass transfer trays and a liquid stream descending in the separating column and comprising dissolved polymerization inhibitor, at least one of said streams comprising (meth)acrylic monomers, and the inner surface of the separating column being sprayed with the liquid stream descending in the separating column and comprising dissolved polymerization inhibitor, and the separating column having internals of whose surface at least parts are in the shadow region of the sprayed descending liquid stream.
2. Description of the Background
In this document, the notation (meth)acrylic monomers is an abbreviation of “acrylic monomers and/or methacrylic monomers”.
In this document, the term acrylic monomers is an abbreviation of “acrolein, acrylic acid and/or esters of acrylic acid”.
In this document, the term methacrylic monomers is an abbreviation of “methacrolein, methacrylic acid and/or esters of methacrylic acid”.
In particular, the (meth)acrylic monomers addressed in this document are intended to include the following (meth)acrylic esters: methyl acrylate, methyl methacrylate, n-butyl acrylate, iso-butyl acrylate, iso-butyl methacrylate, n-butyl methacrylate, tert-butyl acrylate, tert-butyl methacrylate, ethyl acrylate, ethyl methacrylate, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, N,N-dimethylaminoethyl acrylate, N,N-dimethylaminoethyl methacrylate, cyclohexyl methacrylate, 1,4-butanediol monoacrylate, hydroxyethyl acrylate, hydroxyethyl methacrylate, hydroxypropyl acrylate, hydroxypropyl methacrylate, glycidyl acrylate and glycidyl methacrylate.
As a consequence of their very reactive ethylenically unsaturated double bond, (meth)acrylic monomers are valuable starting compounds for preparing polymers which find use, for example, as adhesives, water-absorbing resin or as a binder for emulsion paints.
(Meth)acrolein and (meth)acrylic acid are prepared on the industrial scale predominantly by catalytic gas phase oxidation of suitable C3-/C4 precursor compounds (or of precursor compounds thereof), in particular of propene and propane in the case of acrolein and acrylic acid, or of isobutene and isobutane in the case of methacrylic acid and of methacrolein. However, other suitable starting substances in addition to propene, propane, isobutene and isobutane are other compounds containing 3 or 4 carbon atoms such as isobutanol, n-propanol or precursor compounds thereof, for example the methyl ether of isobutanol. (Meth)acrylic acid can also be obtained from (meth)acrolein.
A product gas mixture is normally obtained from which the (meth)acrylic acid or the (meth)acrolein has to be removed.
Esters of (meth)acrylic acid are obtainable, for example, by direct reaction of (meth)acrylic acid and/or (meth)acrolein with the appropriate alcohols. However, this case also results initially in generally liquid product mixtures from which the (meth)acrylic esters have to be removed (for example by rectification).
For the aforementioned removals, separating processes are frequently employed which are carried out in a separating column containing a sequence of mass transfer trays as separating internals. In these separating columns, gaseous (ascending) and liquid (descending) streams are in many cases conducted in countercurrent, and, as a consequence of the inequilibrium existing between the streams, there is heat and mass transfer which ultimately results in the desired separation in the separating column. In this document, such separating processes are to be referred to as thermal separating processes.
Examples of, and therefore an element of, the term “thermal separating processes” used in this document, are fractional condensation (cf. DE-A 19924532) and/or rectification (ascending vapor phase is conducted in countercurrent to descending liquid phase; the separating action is based on the vapor composition at equilibrium being different to the liquid composition), absorption (at least one ascending gas is conducted in countercurrent to at least one descending liquid; the separating action is based on the different solubility of the gas constituents in the liquid), stripping (like absorption; however, the liquid phase is laden with a component which is taken up by the stripping gas) and desorption (the reverse process to absorption; the gas dissolved in the liquid phase is removed by partial pressure reduction).
For example, the removal of (meth)acrylic acid or (meth)acrolein from the product gas mixture of the catalytic gas phase oxidation can be carried out in such a way that the (meth)acrylic acid or the (meth)acrolein is additionally basically removed by absorption into a solvent (for example water or an organic solvent) or by fractional condensation of the product gas mixture and the resulting condensate or absorbate is subsequently separated rectificatively (generally in a plurality of stages) to obtain more or less pure (meth)acrylic acid or (meth)acrolein (cf. for example, EP-A 717019, EP-A 1125912, EP-A 982289, EP-A 982287, DE-A 19606877, DE-A 1011527, DE-A 10224341 and DE-A 10218419).
The fractional condensation addressed above differs from the conventional rectification essentially in that the mixture to be separated is fed to the separating column in gaseous form (i.e. fully converted to the vapor form).
Instead of the fractional condensation, a total condensation can initially also be employed and the resulting condensates subsequently separated by rectification.
The gaseous or liquid mixtures which contain (meth)acrylic monomers and have already been addressed may contain the (meth)acrylic monomers either in more or less pure form or in dilution (for example with solvent or with diluent gases). The solvents may be either aqueous or an organic solvent, and the specific type of the organic solvent is substantially insignificant. The diluent gas may be, for example, nitrogen, carbon oxide (CO, CO2), oxygen, hydrocarbon or a mixture of these gases.
This means, for example on the route to obtaining (meth)acryl monomers, thermal separating processes are applied in a highly differing manner to gaseous and/or liquid mixtures whose content of (meth)acrylic monomers may be ≧2% by weight, or ≧10% by weight, or ≧20% by weight, or ≧40% by weight, or ≧60% by weight, or ≧80% by weight, or ≧90% by weight, or ≧95% by weight, or ≧99% by weight.
The (meth)acrylic monomers can accumulate either at the top or at the bottom of the separating column. However, it will be appreciated that fractions containing accumulated (meth)acrylic monomers can also be removed in the upper, lower or middle section of the separating column.
The mass transfer trays present in the separating columns for the thermal separating process fulfill the purpose of providing locations having continuous liquid phases in the separating column in the form of liquid layers. The surface of the vapor or gas stream ascending in the liquid layer and being distributed in the continuous liquid phase is then the decisive exchange surface.
The liquid flows over the mass transfer tray which has a multitude of passages. The gas ascends through these passages, so that the mass transfer process can take place. The reflux liquid is conducted further from tray to tray through the same orifices or through special drain apparatus (downcomers). The latter typically do not fall under the definition of the passage.
One problem area when carrying out thermal separating processes between at least one gaseous and at least one liquid stream, of which at least one comprises (meth)acrylic monomers, is that (meth)acrylic monomers are very reactive with regard to their free-radical polymerization and tend toward undesired polymerization.
It is therefore customary to operate the separating columns with polymerization inhibition. In other words, polymerization inhibitors (e.g. phenolic compounds, amino compounds, nitro compounds, phosphorus compounds, sulfur compounds, N-oxyl compounds and/or heavy metal salts are added to the liquid stream descending in the separating column (referred to in this document as reflux or reflux liquid).
All surfaces of the separating column which are wetted with the liquid stream descending in the separating column are thus automatically polymerization-inhibited.
With regard to the surfaces facing the liquid stream descending in the separating column (for example the upper side of the mass transfer trays), the aforementioned interaction is comparatively problem-free.
However, this no longer applies to those surfaces of the separating column which face away from the descending liquid stream (for example the underside of the mass transfer trays).
On these surfaces, (meth)acrylic monomers present unhibited in the ascending gaseous stream can condense out. The unhibited condensate (the polymerization tendency is particularly marked in the condensed phase as a consequence of the low intermolecular separation) can then polymerize, polymer which forms can accumulate and ultimately make the further operation of the separating column impossible.
EP-A 937488 and EP-A 1044957 therefore describe processes for rectifying mixtures comprising (meth)acrylic monomers, in which the inner surface of the rectification column, including the mass transfer tray understide, is sprayed with polymerization-inhibited reflux via nozzles.
DE-A 10300816 relates to thermal separating processes of mixtures comprising (meth)acrylic monomers, in which the separating column containing a sequence of mass transfer trays is operated in such a way that the gaseous stream moving upward, as it passes through the passages of the mass transfer trays, entrains small liquid droplets of the polymerization-inhibited liquid phase disposed thereon and sprays them upward.
A disadvantage of the procedures of EP-A 937488, EP-A 1044957 and DE-A 10300816 is that they can only achieve surface-covering spraying with polymerization-inhibited reflux at considerable cost and inconvenience.
In other words, there will always be parts of the surface of internals disposed in the separating column which neither face the descending polymerization-inhibited liquid stream nor are covered to a sufficient extent by sprayed polymerization-inhibited reflux.
Such parts of the surface are to be referred to in this document as parts of the surface which are in the shadow region of the sprayed (either via special nozzles and/or via mass transfer trays) descending liquid stream.